This invention relates generally to the field of electrography and more particularly, this invention relates to the fusing of toner images to receivers by means of heat and pressure.
In the field of electrography, as practiced, for example, in commercial copiers, a radiation image of an original to be reproduced is projected upon a uniformly charged photoconductive member to produce a latent electrostatic image corresponding to the original image. A visible toner image is produced by developing the electrostatic image with charged toner particles. If the photoconductive member is reusable in the form of a belt or drum, the toner image is then transferred to a receiver such as a web or sheet of plain paper and fused to the receiver. If the photoconductive member itself is the receiver, then the toner image is fused directly to the member.
One technique which may be used to fuse a toner image to a receiver is through the application of heat and pressure by contacting the toner image with a heated fuser member such as a roller or belt. Commonly, a pair of rollers held together under pressure form a nip through which a toner image carrying receiver is passed. One or both of the rollers are heated to melt the heat softenable toner particles to fuse the toner image to the receiver.
In such fusers, one or both rollers preferably include an elastomeric layer to lengthen the nip through which the toner image passes in order to increase fusing time and to lower fusing energy requirements. The elastomeric layer should be resistant to degradation at high temperatures over a long operating life. In addition, the surface of the fuser roller contacting the toner image should be of a material having good release characteristics to prevent offset of toner particles onto the surface of the fuser roller and to obviate the tendency of the copy sheet to wrap around one of the rollers causing copier malfunction. Although silicone elastomers and fluoroelastomers exhibit good resistance to degradation at high temperatures as well as good release characteristics, it has been found necessary to apply a coating of fuser oil to the fuser roller in order to eliminate any possibility of offset to the roller of contaminants such as toner, paper particles, etc. Thus, various fuser oils such as fluorocarbon oils, silicone oils, and fluorosilicone oils may be applied to the fuser roller to improve its toner offset preventing characteristics.
Additionally, in commercial electrographic copiers which process several thousand copy sheets per hour, it is highly desirable that the fuser be capable of operating over a long life so that the operation of the machine is not interrupted by fuser paper jams or replacement of a defective fuser component. Fusers including a silicone elastomer roller to which silicone fuser oil is applied in combination with a fluoropolymer-coated metallic pressure roller have succeeded in minimizing paper jams caused by toner offset and in increasing the operating life of the fuser. Over a period of time, however, the silicone fuser oil tends to be absorbed into the silicone elastomer causing it to swell. This swell may cause the growth of a step pattern in the roller if copy sheets of a variety of lengths are processed by the copier. These steps are formed by greater swelling due to fuser oil absorption beyond the areas of the roller used to process the shorter length copies. When longer length copies are passed through the nip of the roller fuser, uneven fusing causes image deterioration in the processed copy sheet. Since image deterioration is undesirable, the roller fuser member may have to be replaced necessitating a service call and incapacitating the copier for several hours with attendant inconvenience, frustration and cost. Step growth pattern in elastomeric fuser rollers has been found to be especially persistent when both fuser roller members are provided with elastomeric layers as when processing copy sheets with unfused toner images on both sides of the sheet.
Swelling of a silicone elastomeric layer by absorption of fuser oil may be minimized by providing a multi-layer fuser member wherein the silicone elastomeric layer is overcoated with a layer of material which is resistent to absorption of silicone fuser oil. Such silicone oil resistant materials include the fluoroelastomers such as fluorosilicone elastomer and fluoropolymer-based elastomers such as various vinylidene fluoride-based elastomers which contain hexafluoropropylene as a comonomer, for example, Viton.RTM. A (vinylidene fluoride-hexafluoropropylene) and Viton.RTM. B (vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene) which are available from the Dupont Company. Viton.RTM. is a trademark of DuPont. Such materials are substantially more resistent to silicone fuser oil absorption than silicone elastomer and substantially decrease the forming of steps in the silicone elastomeric underlayer. In order to increase the release characteristics of the fuser roller, a thin coating of silicone elastomer may be applied to the fluoroelastomer to form a three-layered fuser member.
Although the aforementioned multilayer fuser rollers have been found to minimize the formation of steps and consequent image degradation in processed copy sheets, thereby substantially increasing the life of the fuser roller, it has been found difficult to form the multi-layered fuser members due to the difficulty in adhering fluoroelastomers to silicone elastomers. Thus, a fuser roller having a silicone elastomer base layer to which is adhered a fluoroelastomer layer has been found to exhibit separation between the layers with prolonged use. This separation may be accounted for by the lack of affinity of the fluoroelastomers for other materials and by the constant flexing of the fuser roller during use.
As a consequence of the foregoing problems associated with known fuser members, the need has been present for a fuser member to which fuser oil may be applied without swelling of the member and resultant step formation therein. The member should have a long operating life and be able to function at high fusing temperatures without structural failure. A multilayered fuser roller should not have interlayer separation over its useful life.